A darkhound is a "species" of Shadowspawn.
Darkhounds look like very large dogs or wolves and are about the size of a small horse, weighing from 250 - 300 pounds.
They are used by the Shadow to hunt down targets. Their saliva and blood are both poisonous and corrosive, causing bruising and burns. If even a tiny amount of either touches the skin, the victim will die "slowly and in great pain...", and larger amounts will kill a human almost instantly. A bite from a Darkhound is almost invariably lethal. They make no mark on soft ground, but leave prints in stone, and are frequently accompanied by the smell of burned sulfur. In Rhuidean Rand wakes up to "a faint whiff of a foul miasma creeping under the door. Not a smell at all, really; a sense of otherness, but that was how it felt. Rank, like something dead a week in stagnant water." 
Darkhounds are very difficult to kill, people with a very good aim could be able to eliminate some of normal breed trough arrows, before they reach their target. Instead they are not easily killed in short range attacks. Also power-wrought weapons can effectively eliminate them. Balefire is considered one of the few things known to reliably destroy any kind of Darkhound.
Lan Mandragoran claims it is impossible to outrun Darkhounds once they have your scent. However, he adds that they don't like rain, and "a good thunderstorm can stop them completely." Running water will also halt them in their tracks. "They will not usually venture out into the rain, but once running rain fails to stop them".
Darkhounds typically run in packs of ten to twelve, though a pack may be as large as fifty hounds (such as the one encountered by Perrin on the morning he scouts So Habor). Perrin Aybara and Moiraine Damodred fought Darkhounds outside of Illian, and killed one with several arrows to the face.
Though some can be killed with normal weapons, there exists a superior breed of Darkhounds that require balefire to kill permanently and cannot be effectively harmed by other means. If they are damaged by anything that does not completely destroy them, they will simply regenerate. These Darkhounds (or at least a subspecies Rand al'Thor fights at Rhuidean) are seemingly solid but if they are chopped into bits the pieces will liquefy, pool, and reform into the Darkhound once again.
Darkhounds are created by twisting the soul of a wolf. The wolves themselves refer to them as Shadowbrothers. Once the twisted soul of the wolf transformation is complete, it becomes a Darkhound with no memory of its soul's former life. Darkhounds can overpower several large packs of powerful wolves isolatedly-wolves therefore avoid them reluctantly. If Darkhounds consume the soul of a wounded wolf, it becomes a Darkhound.
Masuri Sokawa, an Aes Sedai of the Brown Ajah, has researched Darkhounds and claims to have crossed the paths of seven different packs. She says that the number of packs in existence is in dispute, with some sources saying only seven packs exist, while others claim nine, thirteen, or more. Some ancient sources even state that at the time of the Trolloc Wars, there were "a hundred packs" and "in numbers like unto the nightmares of Mankind." However, given the size of the pack discovered (50+) and the lack of wolves in area, one can only speculate about current or future numbers.
Tales about Darkhounds
Apparently there are stories to frighten children in the southlands saying that Darkhounds run the night in the Wild Hunt, with the Dark One himself the hunter, and crossroads were supposed to be particularly dangerous places to meet them, and the time just after sunset or just before sunrise. 
During the Last Battle in Thakan'dar the blowing of the Horn of Valere is able to summon along the Heroes, also the spirits of all the past dead wolves that were dwelling in Tel'aran'rhiod. They are silvery figures and are among the few things that are able to fight successfully against the Darkhound packs.